Reversal mechanisms and the temperature dependence of σR/σS, H C, and H K in particle assemblies

Hysteresis loops of randomly oriented assemblies of acicular particles were measured at temperatures from 4 to 296 K in maximum fields of 24 kOe. The compositions were γ-Fe2O3, γ-Fe2O3 with 7% Fe2+ (precursor for Co surface treatment), Co surface-treated precursor with 4, 8, and 16 wt. % Co, and Fe. Mean HK ’s were determined from first quadrant data and from susceptibility at remanence. The precursor and the Co surface-treated particles had a similar temperature dependence for HC, σR/σS, and HK. This suggests that the effective Co2+ ions replace Fe2+ cations on the surface. The utility of the different mean HK determinations for indicating the HK distribution width is discussed, as well as the sensitivity to superparamagnetic fractions of the HK determined from susceptibility.